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| Fig. 1: Operation mechanism of metamaterial energy harvesting using electromagnetic and acoustic waves. (Image Source: S. Jeong, after Costanzo and Venneri. [8]) |
Smartphones, essential tools in modern life, consume energy even when the screen is off for background tasks and maintaining Wi-Fi connectivity. Standby energy consumption accounts for 58% of total battery usage. [1] As smartphones integrate with IoT devices, the demand for constant connectivity further emphasizes this issue. Reducing standby energy consumption has potential for extending battery life and lowering overall energy use. This report introduces energy harvesting technology using metamaterials and explores how it can address these challenges.
Metamaterials are engineered structures designed to exhibit unique physical properties, such as negative refraction index or nearly perfect wave absorption. [2] These properties come from engineered internal structures, different from intrinsic material property. These structures typically involve periodic lattice with unit cells smaller than the wavelength of the interacting waves. Within these lattices, wave interactions from different parts of the internal structure combine in a way that the material exhibits desired bulk properties within specific frequency ranges. These unique properties enable applications like wave absorption and energy harvesting from various ambient waves. Fig. 1 shows mechanism of energy harvesting using acoustic and electromagnetic waves. Acoustic waves or electromagnetic (EM) waves are absorbed by the metamaterial. The absorbed energy goes into rectifier or interface circuit for the conversion into DC power or any usable electricity source.
The general wave equation describes wave propagation through a medium and is a second-order partial differential equation. It applies to different types of waves, including acoustic and electromagnetic waves
where ψ represents the wave field (pressure for acoustic waves or electric/magnetic field for electromagnetic waves), and c is the speed of sound for acoustic waves and the speed of light for electromagnetic waves. Maxwells equations govern the behavior of electromagnetic waves, but this wave equation can be derived from them.
When a wave encounters a different medium, it can be reflected, transmitted, or absorbed. The principle of energy conservation states that the incident energy is distributed among these three components
The S-parameters (scattering parameters) are essential in understanding wave behavior in metamaterials. S11 refers to the reflection coefficient, which quantifies the amount of incident wave that is reflected by the material. S21 refers to transmission coefficient. The absorption rate A is related to the S-parameters by
S-parameters, complex numbers that quantify the relationship between the incident and reflected waves, vary with the frequency of the incident wave. The magnitude of S11 is related to the impedance Z and free space impedance Z0 by
The principle behind perfect absorption in metamaterials stems from the impedance-matching mechanism of antennas. By matching its impedance with the target signal to make |S11|2=0, an antenna can efficiently absorb a specific frequency. Within metamaterials, complex interactions occur in the internal structure to control S11 and S21. [3,4] By tuning these parameters, metamaterials maximize absorption within specific frequency ranges.
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| Fig. 2: Example of metamaterial-based perfect absorber. (Image Source: S. Jeong, after Landy et al. [2] |
Electromagnetic metamaterials, also known as left-handed metamaterials, have unique properties such as negative permittivity and permeability. These allow electromagnetic waves to propagate in the opposite direction compared to natural materials. [3]
One notable application is Metamaterial-based Perfect Absorbers (MPAs), as illustrated in Fig. 2. Its unit cell consists of two electric ring resonators connected by an inductive ring parallel. This configuration achieves both electric and magnetic coupling with tunable response by the geometry of the wire. This design effectively minimizes wave reflection, achieving a nearly perfect absorption efficiency of 99% at ω=11.48 GHz and maintaining efficiency above 50% within a 4% bandwidth. [4]
A significant challenge lies in converting the absorbed electromagnetic energy into usable electricity, as additional components are required for energy conversion. [5] Nevertheless, there have been studies for conversion of absorbed electromagnetic waves into DC output. proposed energy harvester is capable of capturing up to 70% of the energy from a planewave having various polarizations and converting it into usable DC power. [6]
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| Fig. 3: Example of acoustic metamaterial designed for energy harvesting. (Image Source: S. Jeong after S. Qi et al. [4]) |
Acoustic metamaterials operate on similar design principles as electromagnetic metamaterials but target different frequency ranges. In addition to minimizing reflection, acoustic metamaterials utilize local resonance to capture energy in the periodic structure. [+6]
One example, shown in 'Fig. 3, involves an array of silicone rubber stubs periodically deposited on a thin 60mmx60mm aluminum plate. When acoustic waves incident from the normal direction, the energy is localized at the center as a form of strain energy. The strain energy is converted to electrical energy by a structure piezoelectric material. This design achieved a maximum output voltage of 1.3 V and a power of 8.8 μW at a bandgap frequency of 2257.5 Hz with 2 Pa acoustic incidence. [4] This frequency corresponds to ambient noise levels commonly found in urban areas or machinery.
While the harvested energy remains small, this example demonstrates the potential for capturing energy from ambient noise. The Wake-Up Receiver (WUR), which continuously listens to radio frequency signals, operates with a current as low as 6.4 μA at 1.5V - 3 μA in standby mode and 3.4 μA during activation. [7] By attaching a metamaterial with a length of √1.2 60 = 65.7 mm, the WUR system can function without a battery, provided the continuous noise level is 2 Pa. This corresponds to approximately 100 dB of noise, which, although relatively high, is feasible in noisy environments such as factories or crowded public spaces like subways. By eliminating the need for a battery, the device not only reduces costs but also avoids common issues with electronic transmission systems and the environmental problems associated with batteries, including toxicity and recyclability concerns.
The growing demand for connectivity among smart devices has amplified the issue of standby power consumption, calling for innovative solutions to achieve sustainable energy usage. Metamaterial-based energy harvesting offers a promising approach by capturing energy from ambient electromagnetic and acoustic waves. Although current technologies generate limited power and face significant commercialization challenges, their potential to enable low-power systems and extend battery life remains significant.
Electromagnetic metamaterials, with their near-perfect wave absorption capabilities, offer applications such as powering low-energy IoT devices. Similarly, acoustic metamaterials show promise in urban settings, where they can convert ambient noise into usable energy. These advancements provide a way for self-powered, battery-free devices that are both cost-effective and environmentally friendly.
While the harvested energy from a single device may be minimal, the cumulative impact across numerous devices in daily life, infrastructure, and industrial applications could be profound. By eliminating the need for batteries, these technologies not only reduce electronic waste but also address critical issues related to toxicity and recyclability. As progress in mechanics, material science, and energy harvesting continues, I believe these technologies can expand to diverse applications in our everyday lives, helping to save energy and promote sustainability.
© Sehui Jeong. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. Wang et al., "Standby Energy Analysis and Optimization for Smartphones," IEEE 7474401, 4th IEE International Conference on Mobile Cloud Computing, Services, and Engineering, 29 Mar 16.
[2] N. I. Landy et al., "Perfect Metamaterial Absorber," Phys. Rev. Lett. 100, 207402 (2008).
[3] M. Amiri et al., "Review on Metamaterial Perfect Absorbers and Their Applications to IoT," IEEE Internet Things J. 8(6), 4105 (2021).
[4] S. Qi et al., "Acoustic Energy Harvesting Based on a Planar Acoustic Metamaterial," Appl. Phys. Lett. 108, 263601 (2016).
[5] Y. Liu and X. Zhang, "Metamaterials: a New Frontier of Science and Technology," Chemical Society Reviews 40, 2494 (2011).
[6] T. S. Almoneef, F. Erkmen, and O. M. Ramahi. "Harvesting the Energy of Multi-Polarized Electromagnetic Waves," Sci. Rep. 7, 14656 (2017).
[7] M. Radfar et al., "Battery Management Technique to Reduce Standby Energy Consumption in Ultra-Low Power IoT and Sensory Applications," IEEE 8847630a IEEE Trans. Circuits Syst. I. Reg. Pap. 67, 336 (2020).
[8] S. Costanzo and F. Venneri, "Metamaterial-Based Energy Harvesting for Wi-Fi Frequency Bands," in Information Technology and Systems: ICITS 2021, Vol. 1, ed. by Á. Rocha et al., (Springer, 2021), p. 452.